I. Field
The present disclosure relates generally to communication, and more specifically to transmission techniques for a wireless communication system.
II. Background
Wireless communication systems are widely deployed to provide various communication services such as voice, video, packet data, messaging, broadcast, etc.
These systems may be multiple-access systems capable of supporting multiple users by sharing the available system resources. Examples of such multiple-access systems include Code Division Multiple Access (CDMA) systems, Time Division Multiple Access (TDMA) systems, Frequency Division Multiple Access (FDMA) systems, Orthogonal FDMA (OFDMA) systems, and Single-Carrier FDMA (SC-FDMA) systems.
A multiple-access system may utilize one or more multiplexing schemes such as code division multiplexing (CDM), time division multiplexing (TDM), etc. The system may be deployed and may serve existing terminals. It may be desirable to improve the performance of the system while retaining backward compatibility for the existing terminals. For example, it may be desirable to employ spatial techniques such as multiple-input multiple-output (MIMO) and spatial division multiple access (SDMA) to improve throughput and/or reliability by exploiting additional spatial dimensionalities provided by use of multiple antennas.
A multi-antenna communication system supports multiple-input multiple-output (MIMO) transmission from multiple (T) transmit antennas to multiple (R) receive antennas. A MIMO channel formed by the T transmit antennas and R receive antennas is composed of S spatial channels, where S≦min {T, R}. The S spatial channels may be used to transmit data in parallel to achieve higher overall throughput and/or redundantly to achieve greater reliability.
An accurate estimate of a wireless channel between a transmitter and a receiver is normally needed at the receiver in order to recover data sent via the wireless channel. Channel estimation is typically performed by sending a pilot from the transmitter and measuring the pilot at the receiver. The pilot is made up of symbols that are known a priori by both the transmitter and receiver. The receiver can thus estimate the channel response based on the received symbols and the known symbols.
The multi-antenna system supports MIMO receivers (which are receivers equipped with multiple antennas). MIMO receivers typically require different channel estimates and thus have different requirements for the pilot, as described below. Since pilot transmission represents overhead in the multi-antenna system, it is desirable to minimize pilot transmission to the extent possible. However, the pilot transmission should be such that MIMO receivers can obtain channel estimates of sufficient quality.
There is therefore a need in the art for transmission techniques to efficiently transmit a pilot in a multi-antenna system that can support spatial techniques while retaining backward compatibility for existing terminals.